The present invention relates to sensors, and more particularly to speed sensors for rotatable members such as shafts.
Sensors for determining angular displacement or speed of rotatable members are known. Often, such sensors are inductive sensors which detect ferromagnetic material, or the absence of such materials, on the rotatable member or a member (e.g., a sleeve) disposed about the member so as to determine angular displacement and thereby speed. However, when the rotatable member displaces radially with respect to its axis of rotation, the sensors may provide a false indication of angular position, which causes error in the speed measurement.
Referring to FIGS. 18-20, one prior art sensor assembly known for at least several years is formed as a combination of sensors for determining radial displacement with respect to a central axis of rotation A and sensors for determining axial displacement along the rotation axis A. Such a sensor assembly may include a plurality of radial displacement sensors SR and a plurality of axial displacement sensors SA, such as for example, eight of each as shown in FIGS. 18 and 21 in a preferred arrangement. Preferably, the eight preferred radial sensors SR are spaced apart circumferentially about a shaft or other member being monitored and the eight preferred axial sensors SA are arranged in four sensor pairs PS (FIG. 20) spaced circumferentially apart about the shaft. Each sensor pair PS includes two axially spaced apart axial sensors SA, such that four axial sensors SA are located on each axial side of the “circle” of radial sensors SR. Further, all four axial sensors SA on each side of the radial sensors SR are wired in series with each other and a separate one of two oscillators for driving the sensors SA, or the same oscillator for driving the sensors SA and SR, as depicted in FIG. 21. As will be apparent to those skilled in sensor technology, the described spacing arrangement and coil combination allows the radial sensors to substantially reject the second and third harmonics of the target and their multiples, and the axial sensors to substantially reject the second harmonic of the target.
As best shown in FIG. 18, in the preferred prior art sensor assembly described above, each one of the plurality of radial and axial sensors SR, SA is preferably an inductance sensor that includes two core members each wound with a separate coil and arranged such that magnetic flux passes out of the end of one core member and into an adjacent end of the other core member (not indicated). Further, the prior art sensor assembly also includes a combined radial/axial sensor target disposed on a shaft and consisting of a ferromagnetic annular member TF disposed axially between a pair of nonferromagnetic annular members TN, as shown in FIGS. 19 and 20. The radial sensors SR are positioned to sense the central portion of the ferromagnetic target member TF and the two axial sensors SA of each sensor pair PS are positioned such that each sensor SA detects a separate one of two opposing axial sides L and R of the ferromagnetic member TF, as shown in FIG. 20.